Parallel reactors are widely used in research into chemical reactions, in particular for high throughput experimentation. In high throughput experimentation, a plurality of relatively small scale reactors is arranged in parallel. In each reactor, a different experiment takes place. Usually, conditions and/or reactants are varied slightly over the different reactors. For example all reactors are operated at the same pressure and temperature, but all contain a different reactant. After the experiments are carried out, the results of the experiments are compared with each other, and for example interesting reactants (e.g. catalysts) are identified. Carrying the experiments out in parallel leads to a significant reduction in the time it takes to come up with experimentation results.
Usually, in high throughput experimentation, the reactors are small, as are the amounts of reactants that are used. Often, flow through reactors are used, and the flow rates of the fluid flows are also low. Typical reactor sizes do not exceed 1 cm in diameter, and when for example catalytic activity is tested, typically a few grams of a potential catalyst are present in each reactor. Sometimes even less potential catalyst is used, e.g between 0.005 and 1 gram. Flow rates are usually less then 10 ml/hour for the combined fluid flow from the source to the reactors for liquids, often even 10 μl/hour or less for the liquid flow through a single reactor, and/or less than 150 Nml/minute for gas flow. The typical low flow rate used in high throughput reactions makes controlling the fluid flow through the individual reactors hard.
In order to be able to compare the results of the experiments that are carried out in the different reactors with each other, it is important to accurately control the process conditions of each experiment. Such process conditions include e.g. temperature, pressure and flow rate.
A different use of parallel reactors in chemistry is production of chemical compounds using microreactors. Microreactors are of similar size or slightly larger than the reactors used in high throughput experimentation. When it has been proven that a certain compound can be produced at a small scale, using a micro reactor, a plurality of such micro reactors is arranged in parallel. The compound is then produced in these microreactors, using the same reaction conditions as in the original, single reactor.
One way of controlling the flow rate to the individual reactors is the use of active flow controllers, e.g. needle valves. Active flow controllers are flow controllers that allow the flow rate to be changed during an experiment, for example by adjusting the resistance to fluid flow of the flow controller during the experiment. This is contrary to passive flow controllers such as capillaries that are operated at a constant temperature, which have a fixed resistance to fluid flow.
However, active flow controllers have several disadvantages. Active flow controllers are generally bulky and expensive, which makes them awkward to use in the reactor feed lines of small parallel reactors. Furthermore, in practice it turns out to be hard to accurately control the flow rate of small fluid flows (as are used in high throughput experimentation) using active flow controllers.
Another disadvantage of active flow controllers is that they required repeated calibration, and that during experiments, in particular during experiments that have a long running time, drift in the settings of the active flow controllers often occurs. Also, active flow controllers comprise a flow sensor and flow sensors are sensitive to drift during the course of a long experiment, more than for example pressure sensors.
WO99/64160 discloses a system and method for performing high throughput experiments. In this known system and method, a plurality of parallel flow-through reactors is applied. These reactors are fed with a reaction fluid that comes from a common fluid source. The reaction fluid coming from the common fluid source is distributed over the parallel reactors.
In the reactors, the reactions produce reactor effluent. The reactor effluent from the reactors is collected in a common exit control volume. Sequentially, samples are taken from the reactor effluent of the individual reactors for analysis so the performance of a potential catalyst can be evaluated.
The pressure in the system of WO99/64160 is controlled by controlling the pressure in the common exit control volume.
In the system and method according to WO99/64160, the flow is distributed substantially equally over the reactors. This is achieved by arranging passive flow restrictors upstream or downstream of each reactor. The passive flow restrictors all have the same resistance to fluid flow. Furthermore, the resistance to fluid flow of the passive flow restrictors is by far the highest resistance to fluid flow of all components in the system. This is done because it is not possible to have the same resistance to fluid flow in all other components of the system. For example, the pressure drop over the parallel reactors, and therewith the resistance to fluid flow of the parallel reactors, is likely to vary somewhat from one reactor to another. By giving the passive flow restrictors a resistance to fluid flow that is far higher than the resistance to fluid flow that can be expected in the reactors, the variation in resistance to fluid flow over the different reactors has little effect on the flow distribution.
However, the resistance to fluid flow of a component is directly linked to the pressure drop over the component. This means that in the system and method according to WO99/64160, a large pressure drop over the flow restrictors is required.
For example, if the pressure drop over the restrictors is 10 bar, and the variation in pressure drop over the plurality over reactors is 0.5 bar (=5% of the pressure drop over the restrictors), the deviation from the equal flow distribution will also be about 5%. If the pressure drop over the restrictors is 100 bar, and the variation in pressure drop over the plurality over reactors is 0.5 bar (=0.5% of the pressure drop over the restrictors), the deviation from the equal flow distribution will be about 0.5%. As a deviation of the equal flow distribution of less than 2%, preferably less than 0.5% is generally desired in high throughput experiments or production with microreactors, the prior art teaches the pressure drop over the restrictors should be much higher than the expected pressure drop over the other components in the system, such as the reactors, any filters if present, the tubing of the system.
However, such a high pressure drop over the restrictors can be problematic.
For example, when a certain reaction pressure is desired, the pressure in the common reaction fluid source has to be rather high. Sometimes such a high pressure in the common reaction fluid source is not available or cannot be obtained. Furthermore, in some cases it is not desirable to have a high pressure in the reaction fluid as this may invoke unwanted condensation of gaseous reaction fluid that is fed to the reactor. Furthermore, all components in the system have to be designed such that they can withstand the high pressures in the system. This makes the system complicated and expensive.
In addition, the pressure drop over any reactor could change in the course of an experiment or production run, for example due to formation of solids (e.g. carbon) and/or highly viscous liquids (e.g. heavy tars), and/or leaching of catalyst material which carbon, tar or catalyst material then accumulates for example in the reactor tube, in the frit of a fixed bed in the reactor if such a fixed bed is present, and/or in a filter in or downstream of the reactor. As sometimes the test runs may last for more than a month or even up to six months, this can be expected to occur regularly. If the pressure drop over the restrictors is to be designed as the highest pressure drop in the system, this possible rise in pressure drop over the reactors has to be taken into account as well. This increases the required pressure drop over the restrictors even more.